The present invention relates to lead-acid batteries and, more particularly, it relates to paste mixes used as active materials in batteries.
From early on in its history, the lead acid rechargeable battery industry has mixed water and/or dilute sulfuric acid when mixing various types of battery-grade, lead-lead oxide powder to form pastes. These pastes are used to make plates which are assembled into a battery. In recent years, increased demand for powders has resulted in changes in powder composition. These changes have forced the battery industry to modify the processes involved in preparing active battery plate pastes from the powders.
Originally, the batch process, used in making battery-grade lead oxides, produced lead oxides known as "Barton Oxides." These oxides had essentially no "free lead," which resulted in a simple curing process for producing the unformed active material paste. As the demand for battery-grade oxides increased it became necessary to speed up and mechanize the process of manufacturing oxides. This mechanization, however, has resulted in the formation of powders having lower lead oxides, and a larger portion (about 15% to 30%) of free lead. The particle size of the powders is either controlled during the oxidation process or arrived at by a separation process.
The manufacture of new battery-grade lead oxides has necessitated new paste mixing and curing processes. Specifically, it has become necessary to allow for a certain curing period after pasting the battery plates, in order to allow for the free lead in the oxide powders to oxidize to a level below 5% and nearer 1%. Failure to oxidize the free lead to below 5% may result in unacceptably formed plates. This is particularly true when using such powders in conventional water and/or acid mixed wastes. The pastes must be cured to avoid this problem. But, the curing process is time consuming and costly. Typically, a curing cycle of up to three days is required to arrive at a free lead content below 5% and preferably not more than 1%. Further, this curing process must be done under rigidly controlled temperature and humidity conditions.
Over the years, many additives and/or expanders have been used in fabricating battery pastes. Some of these additives have been used as bulking agents to maintain uniformity of plate density. Some have been used to optimize active material porosity, which may range from 35% to 50%. This is roughly equivalent to a paste density range of 79 to 62 g/in.sup.3 (respectively) with conventional acid base pastes. Densities less than this in positive plate use tend toward excessive paste shedding and reduced life.
Among the additives that have been used in battery pastes is one described in U.S. Pat. No. 4,315,829 to Joseph C. Duddy et al. This patent describes the use of polyfluoroethylene as an additive, used in mixing leady oxides with water and 1.400 s.g. acid. A specific additive mentioned there is Teflon 30TFE (1/8 to 5%).
In general, battery manufacturers have been able to maintain plate quality in spite of changes in the composition of lead oxide powders. As a consequence, the improvements made in the energy density capabilities of lead-acid batteries have not come from plate composition. Instead, these improvements have come from physical changes such as lighter case materials and improved connectors to reduce overall weight.